The coagulation of tissue, particularly the sealing or occlusion of vessels such as veins and arteries is an important feature of many surgical interventions. There is a range of equipment, both mechanical and electrosurgical, for sealing or clamping vascular tissue. In the electrosurgical field, it is typical for a jawed instrument to grasp tissue and deliver a coagulating radio frequency (RF) current. Prior art patents such as U.S. Pat. No. 6,893,435, and U.S. Pat. No. 6,743,229 disclose examples of this type of system. The subject matter of U.S. Pat. No. 6,893,435 is hereby specifically incorporated in the present application by reference.
It is essential to apply the coagulating current signal for sufficient time to enable the tissue to be fully coagulated, but it is not desirable to continue to apply current excessively after the time that the tissue is coagulated. Excessive application of a coagulating waveform will produce desiccation of the tissue, and it may become adhered to the jaws of the instrument, resulting in a tearing of the tissue when the jaws are removed. There have been numerous attempts to predict when the coagulation process has been completed, and advise the user of the system to end the application of power to the tissue. Some systems automatically stop the supply of RF power after a certain criteria has been reached. Examples of these types of systems include U.S. Pat. Nos. 5,423,810, 5,540,684, 5,827,271, 6,228,080, 6,648,883, 6,730,080, and 7,172,591. However, due to the variation between different samples of tissue being coagulated, both in terms of thickness and composition, each of the above systems has its own limitations. The present invention attempts to provide an improvement to this type of system, capable of dealing with a wider range of different tissue types and circumstances.